The highly varied class of cellulose ethers, including the group of binary alkylhydroxyalkylcelluloses including the commercially available representatives methylhydroxyethylcellulose (MHEC) and methylhydroxypropylcellulose (MHPC) has been a field of academic and industrial activity for a number of decades and has been widely described. An overview of the chemical fundamentals and principles of their preparation production processes and process steps) and a listing of materials and description of the products and possible applications of the various derivatives are given, for example, in Houben-Weyl, Methoden der Organischen Chemie, Makromolekulare Stoffe, 4th edition, Volume E 20, p. 2042 (1987)
The processes described and employed for preparing alkylhydroxyalkylcelluloses, e.g., methylhydroxyethylcellulose and methylhydroxypropylcellulose, are based either on a heterogeneous reaction (multiphase reaction mixture) or homogeneous reaction (e.g. single-phase solution). The process itself can be carried out either batchwise or continuously. Heterogeneous reactions are additionally subdivided into gas-phase processes (without liquid reaction medium) and slurry processes (in the presence of a liquid reaction medium).
All process variants for preparing alkylhydroxyalkylcelluloses, e.g., methylhydroxyalkylcelluloses, which have been described and implemented industrially are generally based on the following chemical reaction scheme: In a preceding substep, the cellulosic starting material is activated, preferably by means of aqueous alkali metal hydroxide solution. The alkali metal salt of cellulose which is formed is subsequently reacted with the appropriate alkylene oxide and methyl chloride, with any excess alkali used advantageously being largely neutralized by superstoichiometric amounts of methyl chloride. In the subsequent purification step, salt formed and other by-products are separated off, preferably by washing with hot water.
DE-A 2402740, U.S. Pat. No. 2,949,452 and EP-A 134465 describe gas-phase processes for preparing MHPC in which no liquid or condensed media are present during the etherification reaction. In these processes, the substitution (DS and MS) can advantageously be varied over a wide range. However, as a result of the absence of a liquid heat transfer medium, the exothermic nature of the chemical reactions can not be satisfactorily controlled. In addition, distribution of alkali used and the reactants is a problem. This results in only a moderate level of reproducibility with regard to substitution, and in an uncontrolled and quite substantial decrease in the molecular weight, and thus a fluctuating property profile of the resulting products. In addition, high-viscosity products cannot be obtained by the gas-phase process because of the substantial degradation of the molecular weight.
The problems mentioned in the case of the gas-phase process occur to a much lesser extent, if at all, in the presence of a liquid reaction medium. In the slurry process, inert organic solvents, the reactant methyl chloride in excess or appropriate mixtures thereof usually function as the dispersion medium and heat transfer medium. The reaction medium which is present during the activation and reaction phase achieves, firstly, more uniform alkalization of the cellulose and better transport of the reactants into the alkali metal salts of the cellulose, more uniform substitution with higher reproducibility and chemical yield. Secondly, effective heat removal makes the process more readily controllable overall and significantly suppresses molecular weight degradation by avoidance of local overheating, so that even very high-viscosity products are obtainable. Owing to these process and product advantages, industrial production processes predominantly involve the use of a slurry process.
A disadvantage of the standard slurry process in which the total amounts of the reactants alkylene oxide and methyl chloride are present together during the etherification phase is the restricted ability to adjust the MS. For example, in the case of methylhydroxypropylcellulose, a formally parallel reaction of the reactants results in products having a high DS (methyl) and a low MS (hydroxypropyl). The converse product variant, i.e., a high MS (hydroxypropyl) and intermediate to low DS (methyl), is not obtainable in this type of process even by increasing the amount of propylene oxide employed because of the reaction kinetics. However, such highly propoxylated MC derivatives are of particular interest because of a series of substance-specific properties.
According to U.S. Pat. No. 4,096,325, highly propoxylated MC derivatives can be prepared when the hydroxypropylation and the methylation are carried out separately. As such a variant, EP-A 567869, for example, describes the stepwise reaction of the alkali metal salt of cellulose firstly with propylene oxide, and subsequently in a solvent with methyl chloride. In this way, the DS and the MS can be varied in a targeted manner over a wide range.
In the above-mentioned process variants with stepwise reaction, the hydroxypropylation is generally carried out at high temperatures. In contrast, the methylation, which is strongly exothermic per se, is carried out with cooling at relatively low temperatures. Owing to the long process times and the countercurrent energy flow, these processes have little industrial utility for economic reasons. In addition, increasing separation of the reaction steps results, in a manner similar to the gas-phase process, in problems with uniformity and reproducibility of the substitution, temperature control and molecular weight degradation (viscosity yield).
Owing to the abovementioned disadvantages of the process variants which have been developed and described to date, there is a continuing need for a process which combines the process engineering and product advantages and also the economic advantages of a slurry process with the flexibility of the gas-phase process in respect of achievability and proportion of the MS and DS over a wide range.